The sister group of tetrapods23 is now known to include fish with
pectoral fins that can assume both fin-like and limb-like postures. In
a quasi-planar position, with minimal flexion of the antebrachium,
the fin approximates a generalized posture for a sarcopterygian such
as Eusthenopteron9 (Fig. 7a, c). But the fin could also assume a posture
appropriate for a substrate-supported, upright stance by flexing the
shoulder and elbow and extending the proximal and distal interradial
joints (Fig. 7b, d). Multiple features enable the fin to prop the
body in a limb-like manner: the base of the fin is positioned near the
ventral surface of the body; glenohumeral architecture and transcoracoid
musculature augment flexion and stability at the shoulder
joint; a broad and deep posterior glenoid allows transmission of
substantial propulsive stresses through the pectoral girdle; a robust
coracoid plate provides broad areas for flexor muscle origins; elaborate
ventral processes on the humerus represent extensive surface area for
flexor insertions; flexion/extension, pronation/supination and
rotation are possible at the elbow; and there is an expanded series of
mobile proximal, intermediate and distal radials distal to the epipodials.
Notably, the highly mobile yet robust distal fin segments could
provide a stable but compliant extremity that could conform to
complex and varied substrates.